The hazards of nitrogen oxides and their role in the formation of acid rain and tropospheric ozone have resulted in the imposition of strict standards limiting the discharges of these chemical species. To meet these standards, it is generally necessary to remove at least part of these oxides present in the exhaust gases from stationary or mobile combustion sources.
Denitrification or selective catalytic reduction (SCR) technology is commonly applied to combustion-derived flue gases for removal of nitrogen oxides. The denitrification reaction comprises the reaction of nitrogen oxide species in the gases, such as nitrogen oxide (NO) or nitrogen dioxide (NO2), with a nitrogen containing reductant, such as ammonia or urea, resulting in the production of benign diatomic nitrogen (N2) and water.
In a typical application, catalyst responsible for conducting the reduction of nitrogen oxides is installed in the path of the flue gas flow and ammonia is injected upstream of the catalyst. The SCR system, including the housing, catalyst bed, ammonia injection system controls, and mixing and/or flow control devices, is designed to remove a certain amount of NOx, while staying within a specified maximum level of NH3 slip downstream of the catalyst, a maximum level of oxidation of SO2 (sulfur dioxide) to SO3 (sulfur trioxide), and a maximum pressure loss across the catalyst. Ammonia slip, as used herein, refers to the amount of ammonia present in the exhaust gas stream at the outlet of a catalyst.
Sulfur dioxide oxidation is an undesirable side reaction promoted by SCR catalysts. The maximum limit for sulfur dioxide oxidation is specified to minimize the increase in sulfur trioxide downstream of the catalyst. Increased levels of sulfur trioxide can contribute to fouling of downstream equipment, can result in a visible plume at the exhaust stack of the combustion facility and increase particulate matter emissions.
Similarly, a maximum level of ammonia slip is specified to limit emissions of ammonia from the exhaust, and also to prevent the formation of ammonia salts, such as ammonium bisulfate that result from reaction between ammonia and other flue gas components such as sulfur trioxide. Ammonia salts can foul downstream equipment.
In SCR processes, the reaction between NOx and NH3 generally occurs at a stoichiometry of one mole of NOx per one mole of ammonia:4NO+4NH3+O2→4N2+6H2O  (1)2NO+2NO2+4NH3→4N2+6H2O  (2)
The reaction is not thermodynamically limited at typical SCR temperatures, so if the molar ratio of NH3 to NOx (or NH3/NOx) at the catalyst inlet is 1.0 and a sufficient amount of catalyst is present, the reduction of NOx and consumption of ammonia both approach 100%, and there is essentially no NOx and no ammonia slip at the outlet of the catalyst. If the molar ratio is less than 1.0 and a sufficient amount of catalyst is present, then the NOx reduction efficiency approaches the molar ratio and the ammonia slip approaches zero. If the molar ratio is greater than 1.0 and a sufficient amount of catalyst is present, then NOx reduction efficiency approaches 100% and all excess ammonia slips past the catalyst.
In actual applications, however, it is difficult to attain a consistent molar ratio of NH3/NOx across the entire catalyst bed. There is typically a heterogeneous distribution of NH3/NOx across the catalyst bed stemming from several factors including a heterogeneous distribution of NOx in the exhaust gas, addition of the ammonia at discrete points upstream of the catalyst bed as well as imperfect mixing of the NH3 and NOx downstream of these addition points.
Several techniques currently exist for reducing heterogeneous distributions of NH3/NOx across a catalyst bed. Ammonia flow to discrete points, for example, can be biased to match the molar flow of NOx as determined by the NOx concentration and flow at a particular section. Sufficient mixing length and static mixers are additionally employed downstream of the ammonia injection points in order to obtain mixing prior to the catalyst bed. With these measures in place, in some cases, NOx reduction efficiencies of 93% and 3 ppmvd ammonia slip have been achieved.
Achieving higher NOx reductions (i.e. >95%), nevertheless, remains limited by the heterogeneous distribution of the NH3/NOx molar ratio across the catalyst bed. NH3/NOx molar ratios in excess of equivalence (1.0) can be run to obtain high NOx reduction efficiencies. NH3/NOx molar ratios in excess of equivalence, however, result in undesirable ammonia slip. Moreover, attempts to limit such slip with additional catalytic structures can result in undesirable oxidation of sulfur dioxide.
In view of these problems, it would be desirable to provide compositions and methods for removing nitrogen oxides from exhaust gases operable to achieve high efficiencies while minimizing ammonia slip. It would additionally be desirable to provide compositions and methods for removing nitrogen oxides from exhaust gases operable to achieve high efficiencies while minimizing ammonia slip and sulfur dioxide oxidation.